Project summary The objective of this study is to define associations between gut microbiota, short chain fatty acids (SCFAs) and obesity in populations spanning the epidemiologic transition, and explore mechanisms by which these factors may independently and collectively influence the development of obesity. The gut microbiota and SCFAs have been associated with obesity, yet the causal mechanisms are unknown, as are the individual obesogenic effects of the individual SCFAs (butyrate, acetate and propionate). Existing studies are, limited by contradictory findings, small sample sizes, limited and imprecise measurements of obesity, and lack of detailed dietary and other environmental exposures/mediators. We propose to overcome these challenges by leveraging an existing cohort of five diverse, well-defined populations from the Modeling the Epidemiologic Transition Study (METS, R01-DK080763). METS is comprised of a cohort of 2,500 African-origin adults, living in 5 distinctly different environments; Ghana, South Africa, Jamaica, the Seychelles and the US, and who have been prospectively followed since 2010. Our preliminary data suggest that while gut microbiota and SCFAs differences exist across sites, similar relationships exist across the sites for gut microbiota/SCFAs adiposity effects. In addition to yearly health measurements; we propose to measure gut microbiota and stool SCFAs in all participants (2500) during the first year of the current study, thus providing one of the largest gut microbiota population-based studies to date. We will divide our cohort of 2500 individuals into 2 sets: (1) a test set of 1000 individuals to explore which gut microorganisms and stool SCFAs are associated with adiposity; (2) a validation set of 1500 individuals to independently verify the biomarkers identified in the test set, thus minimizing spurious correlations due to large number of features (e.g., bacterial taxa). We will follow all 2500 individuals for 3 years to assess weight and adiposity changes, using Bayesian Kernel Machine Regression modeling to explore whether changes can be predicted by gut microbiota and SCFAs factors. Finally, using a causal mediation analysis, we will identify the direct and indirect effect of single and/or cumulative gut microbiota on adiposity as mediated by SCFA. We will thus capitalize upon an existing, extensively well described cohort of adults of African-origin, with significant variability as a result of the widespread geographic distributions, and therefore variation in the environmental covariate exposures. The proposed study will substantially advance the understanding of the role gut microbiota and SCFAs play in the development of obesity and provide novel obesity therapeutic targets targeting SCFAs producing features of the gut microbiota.